This invention relates to window or door intrusion sensors for security systems, and in particular to a wireless sensor, contained in a singular housing, that detects a change in the position of a door or window to which the sensor is attached by detecting a change in the magnetic field created by the earth.
Conventional door or window sensors in security systems contain two housings; one housing with a magnet, and one housing with a sensor such as a reed switch, which is a miniature encapsulated switch that is activated by a magnetic field. One of the housings is mounted to the door or window (entrance closure) being monitored and the other housing is mounted to the doorjamb or windowsill associated with the entrance closure being monitored. When the entrance closure is closed and the magnet is in close proximity to the reed switch sensor, the sensor produces an output signal that indicates that the door is in its closed position. Once the entrance closure is moved the magnet is not in close proximity to the reed switch sensor and the sensor produces an output signal that indicates the door is not in its closed position. The output signal is periodically read by the alarm system controller, and when the signal indicates that the door is not in its closed position, the alarm system controller activates an alarm condition. The alarm system controller may receive this information through wired or wireless transmission. Alarm systems of this type are described in U.S. Pat. Nos. 4,677,424; 4,339,747; 3,896,427; 3,668,579; 4,359,719; and 4,241,337.
Alarm systems using reed switch sensors, as described above, are reasonably successful in many applications, although there are a number of drawbacks as follows:
1) There is additional cost and time, during installation, for the installer to mount a second device (i.e. the magnet).
2) The position of the magnet in conjunction with the sensor is often critical and the installer spends time shimming and locating the magnet to optimize the reed to magnet gap.
3) Reed switches, which are glass encapsulated switches are fragile and may be damaged at the time of installation.
4) Sensors with two housings can be defeated during the period when the system is in the disarmed state by the addition of an extra magnet taped to the sensor housing. This maintains the reed in its closed position even if the door is opened during an armed state.
It is therefore an object of the design to deliver improved security to the system, since any attempt to tamper with the device by adding a magnet would cause an alarm condition.
It is therefore an object of the present invention to provide an entrance closure sensor that is contained in a single housing.
It is a further object of the present invention to provide an entrance closure sensor with improved sensor reliability.
In accordance with these and other objects, the present invention is a method, an apparatus, and a system for detecting a change in position of an entrance closure in an alarm system, wherein the entrance closure is either a door or a window.
The method comprises the steps of attaching a singular housing on an entrance closure; monitoring, with apparatus in the housing, the magnetic field of the earth, detecting a change in the position of the housing with respect to the magnetic field of the earth, generating an alarm signal upon detecting a change in position of the housing with respect to the magnetic field of the earth that exceeds a first predetermined threshold, and transmitting by wireless transmission the alarm signal to a remote receiving station.
The apparatus comprises a singular housing with means for monitoring the magnetic field of the earth, means for detecting a change in the position of the housing with respect to the magnetic field of the earth, means for generating an alarm signal upon detecting a change in position of the housing with respect to the magnetic field of the earth, and means for transmitting by wireless transmission the alarm signal to a remote receiving station. The alarm signal may contain a programmable unique transmitter identification number that allows the receiving station to decipher which sensor has sent the alarm message. The monitoring of the magnetic field of the earth is performed by a magnetometer that senses the earth""s magnetic field and generates an output signal correlated to the earth""s magnetic field. A microprocessor detects a change in the position of the housing by sampling the magnetometer""s output signal at predetermined intervals and determining if the sampled output is different from a stored static (initial) output. If the sampled output is different from the stored static output by a first predetermined amount the microprocessor generates an alarm signal and causes the alarm signal to be transmitted. In addition, if the sampled output is different from the stored static output by a second predetermined amount, the microprocessor generates a trouble signal, wherein the second predetermined amount is less than the first predetermined amount. This may occur when the door or window is slightly ajar. This feature is useful to a user during arming of the alarm system, wherein the user can ensure the entrance enclosures are closed prior to vacating the premises being monitored.
The alarm system comprises the apparatus described above for detecting a change in position of an entrance closure, and a receiving station, located remotely from the apparatus. The receiving station comprises means for receiving by wireless transmission the alarm signal from the apparatus, and means for indicating an alarm condition in response to the receipt of the alarm signal.
The step of detecting a change in the position of the housing with respect to the magnetic field of the earth may take place on three axes and the generation of an alarm signal may occur upon detecting a change in position of the housing in two of the three axes.
In order to provide security with less false alarms, the remote receiving station may correlate the alarm signal from the apparatus of the present invention with a second alarm signal from a different sensor, which may be a motion sensor.